1. Field of the Invention
The present invention relates to a light-emitting element of a current excitation type. Moreover, the present invention relates to a light-emitting device and an electronic device each having the light-emitting element. In more detail, the present invention relates to a long-life light-emitting device which is superior in color purity. Further, the present invention relates to a long-life light-emitting device and electronic device which are superior in color purity.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. As a basic structure of these light-emitting elements, a layer containing a substance with a light-emitting property is interposed between a pair of electrodes. By application of voltage to this element, light emission from a substance with a light-emitting property can be obtained.
Since such a light-emitting element is a self-luminous type, there are advantages such as higher visibility of a pixel than visibility of a liquid crystal display, and unnecessity of a backlight. Accordingly, such a light-emitting element is considered to be suitable as a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light, which is a great advantage. Moreover, the light-emitting element has a feature that response speed is extremely fast.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by formation of a large-area element. This characteristic is difficult to be obtained by a point light source typified by an incandescent lamp or an LED, or a line light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a plane light source that can be applied to lighting or the like.
Although the light-emitting elements using electroluminescence are classified roughly in accordance with whether they use an organic compound or an inorganic compound as a substance having a light-emitting property, in the present invention, an organic compound is used for the substance with a light-emitting property.
In that case, by application of voltage to the light-emitting element, electrons and holes are injected from the pair of electrodes into the layer containing an organic compound with a light-emitting property to cause current flow. Then, by recombination of these carriers (electrons and holes), the organic compound with a light-emitting property forms an excited state, and light is emitted when the excited state returns to a ground state. Because of such a mechanism, this kind of light-emitting element is referred to as a light-emitting element of a current excitation type.
It is to be noted that an excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In order to overcome many problems derived from materials of such a light-emitting element and to improve its element characteristics, improvement of an element structure, material development, and so on are carried out.
For example, Patent Document 1 (Patent Document 1: Japanese Published Patent Application No. 2000-068057) discloses a long-life organic EL element in which two light-emitting layers are included to be doped with plural kinds of fluorescent substances having different colors, whereby color stability is secured. However, in Patent Document 1, since doping with plural kinds of fluorescent substances having different colors is performed, light emission with good color purity cannot be obtained, even though white light emission with good color stability can be obtained.
Further, a light-emitting element using an organic compound with a light-emitting property can be driven at lower voltage compared with a light-emitting element using an inorganic compound with a light-emitting property. However, there is a problem that the element has the short life. Accordingly, the light-emitting element is desired to have the longer life.